Handheld thermal imaging cameras, for example, including microbolometer detectors to generate infrared images, are used in a variety of applications, which include the inspection of buildings and industrial equipment. Many state-of-the-art thermal imaging cameras have a relatively large amount of built-in functionality allowing a user to select a display from among a host of display options, so that the user may maximize his ‘real time’, or on site, comprehension of the thermal information collected by the camera.
As is known, infrared cameras generally employ a lens working with a corresponding infrared focal plane array (FPA) to provide an image of a view in a particular axis. The operation of such cameras is generally as follows. Infrared energy is accepted via infrared optics, including the lens, and directed onto the FPA of microbolometer infrared detector elements or pixels. Each pixel responds to the heat energy received by changing its resistance value. An infrared (or thermal) image can be formed by measuring the pixels' resistances—via applying a voltage to the pixels and measuring the resulting currents or applying current to the pixels and measuring the resulting voltages. A frame of image data may, for example, be generated by scanning all the rows and columns of the FPA. A dynamic thermal image (i.e., a video representation) can be generated by repeatedly scanning the FPA to form successive frames of data. Successive frames of thermal image data are generated by repeatedly scanning the rows of the FPA; such frames are produced at a rate sufficient to generate a video representation of the thermal image data. Individual pixels have unique response characteristics. These non-uniformities often result in fixed pattern noise. Many infrared cameras have functionality to provide the ability to correct for this. For example, some infrared cameras can automatically or manually perform offset compensation, which corrects for variations in the individual pixel responses by observing a uniform thermal scene (e.g., by placing a shutter between the optics and the FPA) and measuring offset correction data for each pixel which provides the desired uniform output response. These measured offset corrections are stored, then later applied in subsequent infrared measurements (e.g., with the shutter open) to correct for fixed pattern noise. Other compensations can also be applied, such as 2-point correction.
Temperature changes within or surrounding infrared cameras are found to result in the individual pixels further exhibiting their unique response characteristics. In particular, the change in temperature of the camera's internal components, e.g., due to self-heating or as the result of changes to the surrounding ambient temperature, leads to the individual pixels exhibiting fixed pattern noise over extended lengths of time. For example, during initial powering of an infrared camera, the internal components can be found to continue to rise in temperature for a period of time before the camera becomes thermally stable.
As is known, offset compensation functionality is found in most conventional infrared cameras because it leads to improved imaging capabilities. During the period when the shutter is placed between the optics and the FPA, thermal scene energy is focused on the shutter by the optics. Such energy may heat the shutter. In addition, the solenoid that actuates the shutter between its open and closed positions may heat up the shutter if the solenoid is frequently used. A change in shutter temperature may negatively impact the offset compensation functionality, resulting in poor imaging capabilities. It is believed that past infrared cameras have not accurately tracked changes in the shutter temperature.
Infrared cameras have often been used as radiometers to measure the temperature of objects or targets. Among other uses, these instruments are frequently used in industrial applications as part of a predictive maintenance program. These types of programs typically rely on periodic inspections of the assets of a plant or facility to discover likely failures before they occur. Often plant personnel will develop a survey route in order to routinely gather temperature data on the identified equipment. After collecting a baseline for each piece of equipment, or noting the specified operating temperatures, a technician can then identify changes in the thermal characteristics of equipment over the course of several inspections.
The principle of operation of a radiometer is well known. All surfaces at a temperature above absolute zero emit heat in the form of radiated energy. This radiated energy is created by molecular motion which produces electromagnetic waves. Some of the energy in the material is radiated away from the surface of the material. The radiometer is aimed at the surface from which the measurement is to be taken, and the radiometer optical system receives the emitted radiation and focuses it upon an infrared-sensitive detector. The detector generates an electrical signal which is internally processed by the radiometer circuitry (e.g., microprocessor). One or more temperature sensors help establish the absolute temperature of the detected radiation. The detected radiation may then be converted into temperature data, which can then be displayed.
A number of factors can introduce inaccuracies into the temperature measurements. For example, temperature changes within or surrounding infrared cameras are found to affect radiometry operation. In particular, the change in temperature of the camera's internal components, e.g., due to self-heating or as the result of changes to the surrounding ambient temperature, must be accounted for in a radiometry algorithm. One or more temperature sensors may be employed in positions throughout the camera to track the heat flow in or out of the camera. Past infrared camera designs have not provided such sensors in a compact, cost-effective manner.